The present invention relates to a process for producing a phosphorus-vanadium oxide catalyst precursor, a process for producing a phosphorus-vanadium oxide catalyst, and a process for producing maleic anhydride by vapor phase oxidation using the catalyst, and particularly the present invention relates to an improved process for producing a catalyst precursor composed of phosphorus-vanadium oxide for production of a catalyst which is useful in the production of maleic anhydride by vapor oxidation of a hydrocarbon having 4 carbon atoms such as butane, butene and butadiene, and an improved process for producing the catalyst.
Catalysts containing tetravalent vanadium and pentavalent phosphorus have been used for production of maleic anhydride by selective oxidation in a vapor phase of a hydrocarbon having 4 carbon atoms such as butane, butene, butadiene or the like, especially n-butane which is a saturated hydrocarbon. There is much literature on vanadyl pyrophosphate ((VO).sub.2 P.sub.2 O7) known as a crystalline mixed oxide catalyst having excellent catalytic performance (see, for example, Chem. Rev. 88, p. 55-80 (1988) and references cited therein). Concerning synthesis of the vanadyl pyrophosphate, it has been reported that its precursor, phosphorus-vanadium oxide, specifically vanadyl hydrogen phosphate hemihydrate (VOHPO.sub.4 0.5H.sub.2 O) can be converted into vanadyl-pyrophosphate through topotactic transformation on calcination.
Many proposals have been made on the production of the precursor, vanadyl hydrogen phosphate hemihydrate. Most of them employ a process for producing the precursor in an organic solvent, which comprises reducing at least a portion of a pentavalent vanadium compound in an organic solvent and then reacting it with a pentavalent phosphorus compound to obtain a composite oxide of pentavalent phosphorus and tetravalent vanadium.
For example, Japanese Patent Publication (KOKOKU) No. 57-8761 (1982) and U.S. Pat. No. 4,132,670 disclose a process in which vanadium of vanadium pentoxide is reduced to a valence of 4.0 to 4.6 in a substantially anhydrous organic solvent and then reacted with orthophosphoric acid. The orthophosphoric acid source used in each of the Examples of the publications is specifically 100% orthophosphoric acid in isobutyl alcohol or 85% phosphoric acid.
Japanese Patent Publication (KOKOKU) No. 1-50455 (1989) discloses production of a catalyst precursor by heating substantially a pentavalent vanadium compound and a phosphorus-containing compound in a saturated organic solvent. In the Examples, 100% orthophosphoric acid in isobutanol is used.
Japanese Patent Publication (KOKOKU) Nos. 2-97 (1990) and 2-98 (1990) and U.S. Pat. Nos. 4,374,043 and 4,317,778 disclose production of vanadium phosphorus oxide catalyst precursors using a mixed phosphorus source. The "mixed phosphorus source" refers to a mixture of orthophosphoric acid, pyrophosphoric acid and a small amount of triphosphoric acid. Specifically, a mixture of 75-90% by weight of orthophosphoric acid and 10-25% by weight of pyrophosphoric acid is used.
Japanese Patent Publication (KOKOKU) No. 62-61951 (1987) and U.S. Pat. Nos. 4,3.65,069 and 4,448,873 disclose production of vanadium phosphorus oxide catalyst precursors using an organic solvent as reaction medium, wherein a portion of the organic solvent is distilled out from the system during the reaction. The used phosphorus source is 85% orthophosphoric acid or a mixed phosphorus source of a composition such as mentioned above.
Industrially, with respect to the reaction for the production of maleic anhydride by vapor phase oxidation of a hydrocarbon, attention is recently focused on use of fluid bed catalysts in place of conventional fixed bed catalysts, and some proposals have already been made regarding the process for producing fluid bed catalysts. For example, Japanese Patent Application Laid-Open (KOKAI) No. 57-122944 (1982) and U.S. Pat. No. 4,351,773 disclose a process which comprises comminuting the catalyst precursor prepared in an organic liquid, introducing the precursor into water to form an aqueous slurry, and then spray-drying the slurry. As the catalysts usable for the reaction, there are shown a catalyst using a catalyst precursor synthesized by using 100% phosphoric acid-or a mixed phosphoric acid in an organic solvent and a catalyst obtained by mixing the precursor with silica. As means for comminuting the catalyst precursor, ball milling is merely mentioned.
Japanese Patent Application Laid-Open (KOKAI) No. 59-55350 (1984) and U.S. Pat. No. 4,647,673 propose a process for producing the fluid bed oxidation catalyst by densification and comminution of the precursor containing a mixed oxide of vanadium and phosphorus to form the fluidisable particles and then calcining these particles under the fluidization conditions. In the Examples of these publications, it is shown that the comminution of the precursor is effected by a ball mill to form an aqueous slurry, followed by spray drying with or without addition of silica sol. It is taught that catalyst strength is unsatisfactory when the comminution is performed by an air mill.
Japanese Patent Application Laid-Open (KOKAI) No. 60-64632 (1985) discloses a process in which a crystalline composite oxide containing tetravalent vanadium and pentavalent phosphorus and obtained by reacting a pentavalent vanadium compound and a pentavalent phosphorus compound in an organic solvent capable of reducing the pentavalent vanadium to a tetravalent state, as the first component; an aqueous solution containing tetravalent vanadium and phosphorus, as the second component; and silica sol as the third component are mixed to prepare an aqueous slurry followed by spray drying thereof. In the Examples of the publication, the three-component mixed slurry is subjected to wet pulverizing and mixing, then spray dried and calcined to form a fluid bed catalyst.
Although there are the vanadium/phosphorus oxide catalysts produced by the known methods including those mentioned above, it is still desired to establish a maleic anhydride production process which can balance at a higher level, all of the following requirements: (1) high yield of maleic anhydride; (2) high performance at a low reaction temperature and long life of the catalyst; (3) high mechanical strength of the catalyst; (4) good reproducibility of the production process.
Under the circumstance, the present inventors have made extensive studies and have found that a maleic anhydride production process which has a high yield even at a lower temperature and keeps excellent reaction results for longer time, can be provided by using an aqueous phosphoric acid of a specified concentration in the preparation of catalyst precursors. The above-mentioned finding is surprising because as the phosphoric acid, 85% phosphoric acid has been used due to good availability on an industrial scale and low cost, or solid phosphoric acid (e.g. 100% orthophosphoric acid) has been used due to knowledge of skilled persons that an anhydrous medium is preferable in the production of the phosphorus-vanadium oxide catalyst (for example, see U.S. Pat. No. 4,132,670, col. 6, p. 47-54). Further, the present inventors have found that the above four requirements are satisfied at a higher level by dry-pulverizing the said catalyst precursor in a high-speed gas flow. The present invention has been attained on the basis of these findings.